This invention relates to lead-acid cells and, more particularly, to sealed, maintenance-free, lead-acid cells suitable for use in deep cycle applications in which the cells must be rapidly recharged.
The application of a cell or battery determines the way it is recharged. A stationary battery is charged very slowly at voltages a few millivolts above the open-circuit voltage of the battery. An automotive SLI battery likewise is on "float" charge most of its operating lifetime. Both the SLI battery and the stationary battery receive relatively shallow (less than 10% depth of discharge) discharges followed by relatively long charging periods between discharges. Hence, the total ampere-hours of charge to be returned to the battery during charge is small; and with a long charging period, a low charging rate is used.
Deep discharge batteries, such as lift-truck, golf-cart, electric vehicle, and remote photovoltaic system batteries, on the other hand are routinely, often daily, deeply discharged to greater than 50%, more typically, to a 70-80% depth of discharge, or more. Further, since these batteries are used typically on a daily basis, it is quite often necessary to completely recharge these batteries from a deeply discharged condition in less than 10-12 hours. These batteries therefore must be recharged with relatively high charging currents to fulfill both the time and complete recharge requirements of these battery systems.
When a lead-acid battery is recharged, virtually all of the charging current in the early portion of the charge is used to convert the discharged active material into charged active material. Depending on the charging rate, significant charging inefficiency generally appears when the battery is about 80 to 95% recharged. The higher the charging rate the sooner the charging inefficiency will begin. This charging inefficiency typically first appears at the positive electrode in the formation of oxygen gas. The negative electrode is much more efficient at accepting charge than the positive electrode and inefficiency at the negative in the form of evolution of hydrogen gas generally does not appear until the negative electrode is almost 95-100% recharged. These inefficiencies require that, in practice, the battery receive some overcharge, typically 5 to 15%.
In a conventional flooded electrolyte battery, the oxygen gas which is generated at the positive electrode escapes from the battery except for a very small amount which is dissolved in the electrolyte and eventually reacts at the negative electrode. The solubility of oxygen in sulfuric acid is extremely low and hence the faster the battery is being recharged, the greater the amount of oxygen gas which escapes into the atmosphere. For example, flooded electrolyte batteries used in telephone exchanges on a standby basis have an extremely low charging rate (typically lower than the C/120 rate) and oxygen generates so slowly that it can dissolve in the electrolyte and react at the negative electrode to yield a relatively high recombination rate. This is not the case with flooded electrolyte batteries charged in 10-12 hours or less. In this case, the total gas flow out of the battery is equal to approximately 97% of all of the overcharge current, assuming that all overcharge current is used to generate oxygen and hydrogen gas. The remaining 3% represents that oxygen which is dissolved in the sulfuric acid electrolyte and reacts with the sponge lead negative active material. Thus, in a deep cycle, flooded electrolyte battery which is recharged in less than 10-12 hours, it is considered virtually impossible not to lose a measureable quantity of gas.
Most commercial chargers used to recharge batteries or cells employed in deep cycle applications are capable of delivering a certain maximum current on short circuit dependent on circuit design parameters. These chargers have a voltage limit and the current delivered into a battery is dependent on the difference (.DELTA.E) between the charger voltage and the battery voltage. The greater the difference in voltage between the charger and the battery, the higher is the current, up to the maximum output current of the charger. As the battery charges, the voltage difference between the battery and the charger decreases; and, hence, the current is reduced.
A lead-acid cell's voltage approaching the end of charge is determined principally by the negative electrode half-cell potential. The negative half-cell potential at which hydrogen begins to evolve, often referred to as the hydrogen overpotential, is significantly affected by the alloy composition of the negative grid. It is well known that calcium alloys have a higher overpotential for hydrogen than antimony alloys, and hence batteries having negative grids formed from calcium alloys can be charged to a higher voltage before hydrogen gas begins to evolve. Because typical commercial chargers, as well as the chargers in automobiles, charge all batteries to the same voltage, total gas evolved in a calcium battery is less than with an antimonial battery.
It is known that positive grids made using antimony-containing alloys may "poison" an antimony-free negative electrode and reduce the hydrogen overpotential of the negative electrode to that of an antimony negative, thereby causing a loss of the low gassing benefits of the system using antimony-free negative grids.
For many applications, the trend in lead-acid technology is to provide batteries which are maintenance-free, i.e.,--a type of battery which may be operated without adding water to the electrolyte during its recommended life. The life of such batteries is limited by the water loss due to gas evolution; and, therefore, excess electrolyte must be used to compensate for the water loss which occurs so as to provide a satisfactory life.
Typically, such cells or batteries have minimized the loss of water by using grid alloys having high hydrogen overpotential. Rigid, self-supporting, and sometimes structurally reinforced grids may be employed, using a variety of either antimony-free, or relatively low antimony, lead alloys. Examples of grid alloy systems used include calcium-lead, calcium-tin-lead, cadmium-antimony-lead, selenium-antimony-tin-lead with various optional alloying ingredients such as silver and arsenic as well as combinations of these alloys.
U.S. Pat. No. 4,166,155 discloses the use of a hybrid construction, utilizing a cadmium-antimony-lead alloy for the positive grids and a calcium-tin-lead alloy for the negative grids to provide a maintenance-free battery with improved water-loss characteristics. This patent likewise makes reference to such alloys as having deep cycle capabilities. The alloy system disclosed has also been used commercially in motive power type cells to reduce the frequency at which water additions are required from about once per week to about once every two months.
Further, lead-acid cells in which the electrolyte is immobilized in gel form are known. Such cells or batteries can provide not only maintenance-free but also spill-free characteristics, viz.--the batteries may be used in any attitude without electrolyte leakage. However, the cracks which develop in the gel during charge, while essential for oxygen transport, result in conditions which shorten cycle life in deep cycle applications.
To provide a sealed design yet avoid the potential problems with gelled electrolytes, sealed systems have been utilized in which electrolyte is immobilized and absorbed in special separators. The separators are not fully saturated, and the gases evolved during overcharge or at other times can diffuse rapidly from one electrode to the other. Thus, under the right conditions, the oxygen that is evolved at the positive electrode can diffuse to the negative electrode where it will rapidly react with active lead. Effectively, this reaction partially discharges the negative electrode, preventing the negative electrode from reaching its fully-charged state so as to minimize the evolution of hydrogen. This sequence results in what has been termed an "oxygen cycle". While the oxygen recombination rate is greater than the rate of oxygen being produced at the positive electrode, there should be minimal water loss and minimal pressure build-up. To allow for relatively rapid recharge following deep discharge, some commercial cells have been designed to operate at relatively high internal pressures (e.g., 25 to 50 psig) to insure an adequate rate of oxygen recombination. The design requirements for this type of cell are electrodes having large surface areas, highly porous, rapidly wetted separators and reduced electrolyte volume.
U.S. Pat. No. 3,862,861 to McClelland and Devitt is an example of a cell configuration recombining oxygen using relatively high internal pressures. The cell having a prismatic container described in Progress in Batteries & Solar Cells, Vol. 2, 1979, pp. 167-170, is an example of a cell operating at low pressures. The former cell utilizes grids that are essentially pure lead, and it can be used in float applications and in deep cycle applications in which limited life is acceptable. The latter cell has calcium-lead alloy grids, is used primarily for float applications, and is not considered suitable for long life, deep discharge applications.
At the present time, sealed lead-acid cells of these types have been available commonly in only small ampere-hour capacity sizes. Usage has thus been generally confined to standby applications such as emergency lighting, alarm systems and limited cycle life portable equipment such as television, lanterns and garden tools. While it has been suggested that a sealed, lead-acid system might be scaled up to larger sizes (Engineering, October 1978, The Age of the Sealed Battery, pp. 1020-22), this has not been successfully accomplished commercially.
In addition, sealed commercial designs typically use either essentially pure lead, as is suggested in U.S. Pat. No. 3,862,861, or lead-calcium grid alloys to take advantage of the high hydrogen overpotential characteristics of these alloy systems. U.S. Pat. No. 3,553,020 to Corbin et al. suggests that antimony be eliminated from the grid alloys used so that the battery evolves less gas during charge and has a lessened tendency for self-discharge. In addition, the use of relatively light weight, thin-wall, plastic battery containers employed in many battery applications has been restricted in sealed systems due to the belief that stronger, substantially more rigid containers are necessary to withstand the internal pressures that can develop. U.S. Pat. No. 3,862,861 thus suggests the use of a release valve such as a bunsen valve which is capable of retaining at least 10 to 15 pounds of internal pressure before venting occurs.
Because all of the commercially available sealed, lead-acid batteries use either a pure lead or a lead-calcium grid system, long life deep-discharge cyclic performance is typically poor. Thus, commercially available cells of the type generally shown in U.S. Pat. No. 3,862,861 are said by their manufacturer to provide a life of about 425 cycles to an 80% depth of discharge with a maximum charge voltage of about 2.43 volts. However, this requires 16 hours for recharge which is too long for practical use in most deep cycle applications requiring high ampere-hour capacities. If the charge voltage is increased to a level typically used for such applications, the life is reduced to about 60 cycles. For such cells, cycle life performance is highly sensitive to charge voltage in the 2.45 to 2.55 volt range that is typical of commercial chargers.
The sealed cell using a prismatic container which was previously described provides a life of about 300 cycles at a 100% depth of discharge. However, as was the case with the cell of the type shown in U.S. Pat. No. 3,862,861, this cycle life is predicted upon the use of a constant voltage recharge at 2.45 volts. Recharging at constant current reduces the life to about 150 cycles or so. These cycle lives are based upon a discharge rate of C/5.
It is well known that the presence of antimony in the positive grid alloy is essential to insure good deep cycle performance. However, antimony-containing positive grid alloys have not been previously used in sealed systems for the reasons set forth in U.S. Pat. No. 3,553,020 to Corbin et al., as previously discussed herein.